The invention relates to fire and explosion detection systems and more specifically to systems which are able to discriminate between fires and explosions which need to be detected and those which do not. For example, systems embodying the invention may be used in situations where it is required to discriminate between (a) a first case where radiation is produced by the explosion or burning of an explosive or incendiary ammunition round striking the protective skin or armor of a vehicle or the like, such as a battle tank, and (b) a second case where radiation is produced by a fire or explosion of combustible or explosive material (such as hydrocarbons) which is set off by such ammunition round. The system is arranged so as to detect the second case but not the first case, and in this way can initiate action to suppress the fire or explosion in the second case but not initiate such suppression action in response to the first case. For example, such a system may be used for protecting regions adjacent to the fuel tanks (and fuel lines and hydraulic systems) in armored vehicles which may be attacked by high explosive anti-tank (H.E.A.T.) ammunition rounds. In such an application, the system is arranged to respond to hydrocarbon fires (that is, involving the fuel or hydraulic fluid carried by the vehicle) as set off by such ammunition rounds, but not to detect either the explosion of the round itself or any secondary non-hydrocarbon fire produced by a pyrophoric combustion of materials from the armor of the vehicle which may be set off by the H.E.A.T. round.
Various forms of such systems have been previously proposed.
One such system is shown in U.S. Pat. No. 3,825,754, Cinzori et al. In the system disclosed by Cinzori et al there are two main channels respectively responsive to radiation (from the source being monitored) in the range of 0.7 to 1.2 microns and in the range of 7 to 30 microns. In the presence of a fire or explosion of the type to be detected, these two channels produce outputs which are fed to a coincidence gate. A third channel has a radiation detector detecting radiation from the source being monitored at 0.9 microns and this channel allows the signals from the two main channels to pass through the coincidence gate only if the energy of the radiation which it detects is less than a predetermined relatively high threshold. The output of the coincidence gate indicates a fire or explosion to be detected. This arrangement is said to discriminate against radiation produced by the explosion or burning of an H.E.A.T. round--which is assumed to produce radiation above the relatively high threshold.
However, such a system, by being dependent for its discriminating action on the level of the energy received in the third channel, is dependent on factors such as the source size and distance.
Another such system is shown in U.S. Pat. No. 4,101,767, Lennington et al. The system disclosed by Lennington et al has a main channel with a radiation detector detecting radiation at 4.4 microns and providing outputs to a logic circuit if the intensity of the radiation which it detects exceeds a predetermined threshold and is rising at at least a predetermined rate. In a subsidiary channel, two radiation detectors, operating at 0.76 and 0.96 microns, produce outputs which are processed to measure the color temperature of the source. If the color temperature exceeds a predetermined relatively high threshold, the logic circuit is prevented from responding to the main channel output. The output of the logic circuit is indicative of a fire or explosion to be detected. This system operates on the basis that an exploding H.E.A.T. round can be discriminated against because its color temperature is very much higher than that of a fire or explosion to be detected.
Such a system is found to be satisfactory but may not discriminate adequately when used in applications where the vehicle armor is non-pyrophoric.
It is an object of the invention to provide an improved fire and explosion detection system. More specific object of the invention is to provide such a system which is better able to discriminate between fires and explosions which are required to be detected and those which are not required to be detected.